Chromating beryllium for high temperature protection



Dec. 23, 1969 F. PEARLSTEIN ETAL 3,485,682

CHROMATING BERYLLIUM FOR HIGH TEMPERATURE PROTECTION Filed Dec. 8. 1966 soo-c CHROMATED J CHROMATED some I 20 4o I00 I20 I40 ex osures 'rms, nouns INVENTOR.

FRED PEARLSTEIN RAYBURN W. WIGK BY ANT ONY GAL ACCIO United States Patent 3,485,682 CHROMATMG BERYLLIUM FOR HIGH TEMPERATURE PROTECTION Fred Pearlstein, Philadelphia, Reyburn W. Wick, Southampton, and Anthony Gallaccio, Havertown, Pa., as-

signors to the United States of America as represented by the Secretary of the Army Filed Dec. 8, 1966, Ser. No. 600,263 Int. Cl. C23-f 7/26 U.S. Cl. 148-62 3 Claims ABSTRACT OF THE DISCLOSURE A method for preventing the high temperature oxidation of beryllium by treating the beryllium with an aqueous solution comprising soduim dichromate and hydrofluoric acid and thereby producing a chromate conversion coating on the surface of the beryllium.

This invention relates to the oxidation of beryllium and more particularly concerns the protection of beryllium against high temperature oxidation.

Because of the desirable properties of beryllium, such as low density, high modulus of elasticity, and high specific heat, the application of beryllium in specialized industrial and military equipment is increasing. In its various uses as a nonstructural material to withstand elevated temperatures, beryllium is limited in serviceability by the fact that it oxidizes catastropically when subjected to temperatures in excess of 700 C. in the presence of moist air.

It has been demonstrated that anodic coatings applied to beryllium from chromic acid solution markedly inhibits the oxidation of beryllium at high temperatures. The degree of protection afforded by the chromic acid anodic coating (in excess of 40 hours at 900 C.) is exceptional in contrast to the rapid oxidation of untreated beryllium.

In the anodic process wherein an electric current is caused to pass through the chromic acid haviing the beryllium suspended therein, an oxide coating is produced on the surface of the metal. Hence, it can be seen that althrough beryllium may be protected from rapid oxidation by anodizing, this process is involved and expensive and because of the low current efliciency of formation, requires a long anodizing time or a high current density. For example, at a current density of 25 amperes per square decimeter a minimum of mintes is required for proper application. A lesser current density would require a greater application time.

It is therefore an object of the invention to provide high temperature oxidation protection for beryllium.

Another object of the invention is to prevent beryllium from being catastrophically oxidized when subjected to temperatures in excess of 700 C. in the presence of moist air.

A further object of the invention is to provide methods for accomplishing the above in a simple and inexpensive manner.

These and other objects of the invention will become apparent upon further disclosure.

Briefly, we have discovered that oxidation protection of beryllium can be achieved when chromate ions are made available at the surface of beryllium as constituents of a chromate conversion film, the film being essentially composed of chromic chromate.

More specifically, chromate conversion coating may be applied to the beryllium by immersion thereof in an aqueous solution of about 200 grams per liter (g./l.) Na Cr O -2H O to which is added from 0.3 to

3,485,682 Patented Dec. 23, 1969 ice 9.6 g./l. of HF (48% Immersion times ranging between about 20 seconds to 9 minutes provide the chromate conversion coating to the beryllium. Longer immersion times may be used for solutions containing less than about 1.8 g./l. of HF (48%). Beryllium specimens having this chromate conversion coating admirably withstands exposure to moist air at high temperatures without significant oxidation. The solution can alternatively be applied to the beryllium by spraying or brushing techniques. Other fluoride containing ions such as fluorosilicates may be used in place of hydrofluoric acid in the chromating solution.

The invention is illustrated by, but not limited to, the following example:

EXAMPLE I Hot-pressed, extruded, and machined beryllium specimens (1.3 cm. diameter x 1.3 cm. length) of 98.4% purity were immersed for 9 minutes in a solution of 200 g./l. Na Cr O -2H O to which was added 1.8 g./l. HF (48% Hot pressed bare beryllium speciments and beryllium specimens chromated in accordance with our invention were subjected to temperatures of 800 C. and 900 C. in a tube furnace through which air, saturated at 25 C. with water vapor, flowed at the rate of 200 ml./min. Specimens were removed from the furnace after being heated for various periods and weighed to determine the extent of oxidation. The results are plotted in the drawing which graphically represents oxygen uptake of bare and chromated beryllium specimens at various temperatures. The chromate film prevented oxidation of the beryllium at least 24 hours at 900 C. The specimen heated for 48 hours revealed two small areas where oxidation had started, the areas enlarging upon continued heating. The bare specimens were visibly oxidized after being heated for only four hours, and continued to oxidize progressively and rapidly upon further heating. At 800 C., the chromated film prevented oxidation of the beryllium for nearly hours.

While the particular process and composition for protecting beryllium surfaces from high temperature oxidation as described herein are well adapted to meet the objects of the present invention, various modifications or changes may be resorted to without departing from the scope of the invention as defined in the claims.

We claim:

1. The process for protecting beryllium surfaces from high temperature oxidation comprising:

contacting said surfaces with an aqueous solution of sodium dichromate and hydrofluoric acid, said solution comprising about 200 g./l. sodium dichromate and about 0.3 to 9.6 g./l. of 48% hydrofluoric acid.

2. The process as described in claim 1 wherein said beryllium is immersed in said solution for a period ranging between about 20 seconds to 9 minutes.

3. The process as described in claim 1 wherein said beryllium is sprayed or brushed with said solution.

References Cited UNITED STATES PATENTS 2,796,371 6/1957 Ostander et a1. 1486.2 3,301,718 1/1967 Morana 1486.2X

OTHER REFERENCES Pearlstein et al., Metal Finishing, January 1966, pp.

RALPH S. KENDALL, Primary Examiner U.S. C1. X.R. 14831.5 

